The present invention relates to a low dislocation substrate useful for epitaxial growth mainly of a GaN system.
GaN has been already put into commercial products as a crystal for short wavelength light elements such as blue and ultraviolet light. Further, studies are being made thereon for developing higher performance elements or on application to electronic devices. However, a GaN-type crystal has no single crystal substrate for its epitaxial growth. Therefore, at the present time, a method of growing a low temperature buffer layer on a sapphire substrate and growing thereon a GaN single crystal layer at a high temperature is mainly used.
The GaN crystal layer grown by this method allows the presence of a very large number of crystal defects due to the great difference in the lattice constant between the substrate sapphire and GaN. The defect density in general is said to be from 108 to 1010/cm2.
When an electronic device is manufactured using such a crystal having many defects, the electronic device is readily deteriorated. This is a serious problem of a laser diode (LD) using a large current.
Accordingly, various attempts are being made to reduce the defects of GaN crystal. One method for reducing crystal defects is transverse growth.
For example, JP-B-6-105797 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d) discloses a technique of forming a mask comprising an insulating material thin film or high melting point metal thin film on a semiconductor substrate such that an exposed area in the shape of a fine line is provided on a part of the mask, and growing a low dislocation epitaxial layer in the direction parallel to the substrate surface by epitaxy over the entire surface of the substrate through the exposed area.
The principle of obtaining low dislocation by this transverse growth is briefly described below. An insulating material thin film or high melting point metal thin film having partially provided thereon exposed areas is formed as a mask on the surface of a semiconductor substrate and an epitaxial layer is grown on this substrate. At this time, by selecting the conditions, the crystal grown using the exposed area as a seed grows in the transverse direction on the surface of the thin film mask part.
The grown layer on the thin film mask does not take over the dislocation directly from the substrate, as a result, an epitaxial growth layer free of or reduced in the dislocation can be obtained.
However, the transverse direction has a problem in that crystal defects are readily generated at the portion where two layers grow in the transverse direction on the mask part surface joining from two adjacent exposed areas.
For example, a case where a GaN layer is grown in the GaN single crystal film on a sapphire substrate using the transverse direction by MOCVD method is reported (see, Applied Physics Letters, vol. 72, No. 2, page 211 (1998)).
In this paper, FIG. 1 at page 211 shows the surface state after an epitaxial growth layer is etched to the depth of 4 xcexcm. In this figure, a large number of linear etch pits are present in the epitaxial growth layer near the center of the SiO2 film mask part. This reveals incomplete joining. On the other hand, the paper emphasizes that by etching of 1 xcexcm, almost no etch pit is generated in the region on the SiO2 film mask part and complete joining is attained.
In either case, it is understood that defects are readily produced in the junction area.
An object of the present invention is to provide a technique of transverse growth, where the insulating material thin film or high melting point metal thin film formed on a single crystal surface is designed to have a specific shape to thereby reduce the crystal defects appearing in the junction area of epitaxial growth layers grown from adjacent exposed areas on the mask part and provide a crystal layer reduced in the defects as compared with that formed by conventional techniques.